The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods and apparatus.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
Typical transmit operations of such a system are illustrated in FIG. 3. Data streams from different users are subjected to various signal processing steps, such as error correction coding or interleaving, and spread using a combination of a user specific spreading code and a group-specific scrambling code. The coded data streams from the users are then combined, subjected to carrier modulation and transmitted as a composite signal in a communications medium.
A so-called Rake receiver structure is commonly used to recover information corresponding to one of the user data streams. In a typical Rake receiver, a received composite signal is typically correlated with a particular spreading sequence assigned to the receiver to produce a plurality of time-offset correlations, a respective one of which corresponds to an echo of a transmitted spread spectrum signal. The correlations are then combined in a weighted fashion, i.e., respective correlations are multiplied by respective weighting factors and then summed to produce a decision statistic.
Several approaches to determining appropriate weighting factors have been proposed. Classical optimal Rake receivers typically are designed with an underlying assumption of uncorrelated noise at the receiver, and thus typically use the complex conjugates of channel coefficients estimated by a channel estimator as weighting factors. Such an approach may yield less than desirable results in CDMA systems, because the passing of interfering signals through the dispersive medium generally introduces correlation into the noise at the receiver. Accordingly, receiver approaches have been proposed based on a model of xe2x80x9ccoloredxe2x80x9d noise, as described, for example, in xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part I: Theory and Background,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 12, pp., 817-827(June 1994); xe2x80x9cA Noise Whitening Approach to Multiple Access Noise Rejection-Part II: Implementation Issues,xe2x80x9d by Monk et al., IEEE Journal on Selected Areas in Communications, vol. 14, pp. 1488-1499 (October 1996); xe2x80x9cData Detection Algorithms Specifically Designed for the Downlink of CDMA Mobile Radio Systems,xe2x80x9d by Klein, 1997 IEEE Vehicular Technology Conference, Phoenix Ariz. (May 4-7, 1997); U.S. Pat. No. 5,572,552 to Dent et al. (issued Nov. 5, 1996); and xe2x80x9cOptimizing the Rake Receiver for Demodulation of Downlink CDMA Signals,xe2x80x9d by Bottomley, Proceedings of the 43rd IEEE Vehicular Technology Conference, Secaucus N.J. (May 18-20, 1993).
Although such approaches can be effective in improving reception of spread-spectrum signals, there is an ongoing need for improved techniques for processing received spread spectrum signals that account for interference from other spread spectrum signals.
In light of the foregoing, it is an object of the present invention to provide improved methods and apparatus for recovering information represented by a spread spectrum signal transmitted in a communications medium.
It is another object of the present invention to provide improved methods and apparatus for recovering information represented by a spread spectrum signal that can compensate for interference from other spread spectrum signals transmitted in the communications medium.
These and other objects, features and advantages can be provided, according to the present invention, by methods and apparatus in which correlations of a received composite signal with a desired spreading sequence are weightedly combined using weighting factors that are generated based on knowledge of the spread spectrum signals present in the composite signal, including pulse shape information, e.g., based on the statistical properties of the desired sequence and power of the interfering spread spectrum signals using other sequences. More particularly, the weighting factors may be generated from a composite channel response estimated using the statistical properties of the desired sequence and an impairment correlation determined from a power estimate of at least one other spread spectrum signal and noise present in the composite signal. According to an aspect of the present invention, updated weighting factors are iteratively estimated from previously computed weighting factors, obviating the need to perform inversion of an impairment correlation matrix.
In particular, according to the present invention, information encoded in a first spread spectrum signal transmitted according to a first spreading sequence in a communications medium is recovered. A composite signal including the first spread spectrum signal is received from the communications medium. The composite signal is correlated with the first spreading sequence to produce a plurality of time-offset correlations of the composite signal with the first spreading sequence. Weighting factors are generated based on knowledge of spread spectrum signals present in the composite signal, including pulse shaping information. The correlations are combined according to the weighting factors to estimate information encoded in the transmitted first spread spectrum signal.
According to one embodiment of the present invention, a composite channel response is estimated from knowledge of the first spreading sequence. An impairment correlation is estimated from knowledge of the first spreading sequence, an estimate of power of a second spread spectrum signal in the composite signal, and an estimate of power of noise in the composite signal. Weighting factors are then generated from the estimated composite channel response and the estimated impairment correlation.
According to another embodiment of the present invention, a multiuser interference correlation and a noise correlation are estimated. The estimated multiuser interference correlation and the estimated noise correlation are then summed to estimate the impairment correlation. An intersymbol interference correlation may also be estimated, and added to the estimated multiuser interference correlation and the estimated noise correlation to estimate the impairment correlation.
According to another aspect of the present invention, weighting factors are iteratively generated from an estimated channel response, an estimated impairment correlation, and previously determined weighting factors. A composite signal including a first spread spectrum signal is received from the communications medium. The composite signal is correlated with the first spreading sequence to produce a plurality of time-offset correlations of the composite signal with the first spreading sequence. The correlations are combined according to the iteratively generated weighting factors to estimate information encoded in the transmitted first spread spectrum signal. The channel response may be a composite channel response estimated from knowledge of the first spreading sequence, and the impairment correlation may be estimated from knowledge of the first spreading sequence, an estimate of power of a second spread spectrum signal in the composite signal, and an estimate of power of noise in the composite signal.
According to another aspect of the present invention, an apparatus for recovering information encoded in a first spread spectrum signal transmitted in a communications medium includes means for receiving a composite signal including the first spread spectrum signal from the communications medium. Means are provided, responsive to the means for receiving, for correlating the composite signal with the first spreading sequence to produce a plurality of time-offset correlations of the composite signal with the first spreading sequence. Means are provided for generating weighting factors based on knowledge of spread spectrum signals in the composite signal, including pulse shaping information. Means are also provided, responsive to the means for correlating and to the means for generating weighting factors, for combining the correlations according to the weighting factors to estimate information encoded in the transmitted first spread spectrum signal.
According to another aspect of the present invention, an apparatus for recovering information encoded in a first spread spectrum signal includes means for iteratively generating weighting factors from an estimated channel response, an estimated impairment correlation, and previously determined weighting factors. Means are provided for receiving a composite signal including the first spread spectrum signal from the communications medium. Means are also provided, responsive to the means for receiving, for correlating the composite signal with the first spreading sequence to produce a plurality of time-offset correlations of the composite signal with the first spreading sequence. Means are also provided, responsive to the means for generating a second set of weighting factors and to the means for correlating, for combining the correlations according to the iteratively generated weighting factors to estimate information encoded in the transmitted first spread spectrum signal.
According to yet another aspect of the present invention, an apparatus for recovering information encoded in a first spread spectrum signal encoded according to a first spreading sequence includes a correlation unit operative to correlate a composite signal with the first spreading sequence to produce a plurality of time-offset correlations of the composite signal with the first spreading sequence. A weighting factor generator is operative to generate weighting factors based on knowledge of spread spectrum signals in the composite signal, including pulse shaping information. A weighted combiner is responsive to the correlation unit and to the weighting factor generator and operative to combine the correlations according to the weighting factors to produce a decision statistic. A detector is responsive to the weighted combiner and operative to generate an estimate of information encoded in the transmitted first spread spectrum signal from the decision statistic.
According to an embodiment of the present invention, the weighting factor generator includes a composite channel response calculator operative to calculate a composite channel response from estimated channel coefficients and knowledge of the first spreading sequence. An impairment correlation calculator is operative to calculate an impairment correlation from estimated channel coefficients, knowledge of the first spreading sequence, an estimate of power of a second spread spectrum signal in the composite signal and an estimate of power of noise in the composite signal. A weighting factor calculator is responsive to the composite channel response calculator and to the impairment correlation calculator to calculate weighting factors from the calculated composite channel response and the calculated impairment correlation.